1. Technical Field
This invention relates generally to lenses, and more particularly to an ophthalmic lens providing astigmatism correction.
2. Background Information
Ophthalmic lenses include intraocular lenses (IOL's), contact lenses, and corneal implant lenses. Each of these has an optical portion with anterior and posterior surfaces, and some means for use in supporting the optical portion on the eye.
An IOL, for example, may include resilient fixation members projecting radially from the optical portion that support the lens within the posterior or anterior chambers of the eye. Contact lenses, on the other hand, have a curved posterior surface which adapts them to be retained by the surface tension of tears, and corneal implant lenses commonly include a peripheral portion that is implanted in a pocket formed in the cornea.
Apart from these differences, the lenses serve a similar function. They help focus impinging light on the retina according to the refractive index of the lens material and the radius of curvature of the lens surface. In doing this, they may sometimes provide astigmatism correction as well.
In many cases, however, an undesirably thick lens is required to produce the optical characteristics desired so that the lens may be thicker or heavier than desired for satisfactory retention on the eye. In addition, high refractive index materials may be less nutrient permeable.
A corneal implant lens, for example, may be implanted by removing a forward portion of the cornea and suturing it back on over the lens. Thus, lightness, thinness, and nutrient permeability are desired. For another example, the implantation of soft IOL's often involves folding the lens to facilitate insertion into the eye and occasional refolding under certain circumstances dictating removal. Thus, thinness for folding purposes is desired as well.
In this regard, German Patent No. DE 3439551 Al describes a one-piece natural lens substitute that can be folded for insertion into the anterior or posterior chamber of the eye. It employs a conventional Fresnel lens pattern that includes individual periaxial annular zones separated by steps. The radii of curvature of the individual zonal areas are selected so that the focal points of all zones coincide, and this results in a thinner, foldable lens.
Although effective in some respects for reducing lens thickness, the Fresnel lens pattern does not provide the astigmatism correction often desired. Thus, there is a need for an ophthalmic lens having attributes of lightness, thinness, and, for some applications, foldability, while providing astigmatism correction.